The invention relates to a projection television display tube comprising in an evacuated envelope a display screen which is provided on the inside of a display tube in the wall of the envelope, said display screen comprising luminescent material and between said material and the display window a multilayer interference filter is provided which comprises a number of layers which are manufactured alternately from a material having a high refractive index and a material having a low refractive index.
From German Patent Application No. 23 30 898, laid open to public inspection, a display tube is known in which a multilayer interference filter is used between the luminescent material (the phosphor) and the display window. For the layers of the filter having a high refractive index, an optical thickness smaller than 0.25.lambda. or between 0.5 and 0.75.lambda. is chosen, wherein .lambda. is the wavelength of the light emitted by the luminescent material. For the layers of the filter having a low refractive index, an optical thickness of 0.25.lambda. is chosen or an odd multiple thereof. The light emission characteristic of the display screen of the tube is varied by the filter in such manner that the quantity of light emitted within a restricted angular aperture increases by 25% compared with a tube without a filter. The contrast in the produced picture is also increased by a reduction of the background brightness.
A similar display tube is disclosed in British Patent Specification No. 1,306,335 in which a so-called passband interference filter is provided between the phosphor and the display screen. This filter consists of layers having an optical thickness of 0.25.lambda. and appears as follows:
S L H L L L L L L H L H L L L L L L H L H L L L L L L H,
wherein S is the display window, L are layers of a material having a low refractive index, and H are layers of a material having a high refractive index. Such a filter may also be represented as follows:
S L H (L).sup.6 H L H (L).sup.6 H L H (L).sup.6 H.
As a result of the presence of the six layers placed against each other and having a low refractive index (L).sup.6, a Fabry-Perot filter is formed. This filter, with a given choice of the optical layer thickness, has a desired transmission band for light rays which enclose an angle smaller than 25.degree. to 40.degree. to the normal on the filter. In addition, however, there is a non-desired wide transmission band for light rays which enclose an angle between 55.degree. and 90.degree.. All the light which is passed in this band is lost or contributes to so-called halo and hence to loss of contrast in the displayed picture. This will be explained hereinafter. Moreover, the cryolite layers used in said filter are hygroscopic and remain soft so that damage of the filter can easily occur. The zinc sulphide (ZnS) layers used in said filter easily oxidize during firing of the tube (at approximately 460.degree. C.), so that they are not so suitable for use in display tubes.
In the article "Anti-halo Coating For Cathode Ray Tube Face-plates", J. D. Rancourt, Proceedings of the SID, Vol. 25/1, 1984, an 11-layer edge-filter and an 8-layer pass-band-interference filter for suppressing halo effects is described. It is also suggested to use one of these filters for projection T.V. (page 47, right-hand top). An 11-layer filter, however, for projection T.V. still gives much loss because too much light is passed at large angles.
It is therefore an object of the invention to provide a projection television display tube having a multilayer interference filter in which the overall quantity of light produced by the tube within an aperture angle of 20.degree.-35.degree. to the normal for the central wavelength is at least 50-60% larger than in such a tube without a filter.
Another object of the invention is to provide a multilayer interference filter such that for light rays which enclose an angle to the normal on the filter between approximately 30.degree. and 80.degree.-90.degree., no second transmission band occurs.
A further object of the invention is to provide a projection television display tube having a multilayer interference filter in which it is possible to realize a colour improvement and a greater contrast in the displayed picture as compared with the known tubes.
Still a further object of the invention is to provide a display tube having a multilayer interference filter which can be fired at 440.degree. to 460.degree. C.